Field of the Invention
The present disclosure relates to an automatic focus (AF) adjustment apparatus and an AF method using phase difference detection and contrast detection, and a photographing apparatus including the AF adjustment apparatus.
Description of the Related Art
One known method to detect a focus in an image pickup apparatus is to detect contrast (hereinafter this method is referred to as contrast autofocus (AF)). Another known method is to detect a focus by detecting a phase different (hereinafter referred to as phase difference AF). The contrast AF is employed in many video cameras and digital still cameras in which an image sensor is used also as a focus detection sensor.
In the contrast AF, an output signal from the image sensor and, more particularly, information on contrast in high frequency components is used to detect a focus lens position at which an AF evaluation value given by contrast information has a maxim value and determine this focus lens position as an in-focus position. In the contrast AF using the above-described scheme, to determine the position of the focus lens and the movement direction of the focus lens to focus on an object where best focus is achieved on an object, it is necessary to acquire contrast information while moving the focus lens. Thus, it may take a long time to achieve an in-focus state, or there is a possibility that the focus lens may be moved into a wrong direction, or the focus lens may be moved further beyond an in-focus position, or other low-quality behaviors may occur.
On the other hand, in the phase difference AF, an amount of defocus is detected from a difference (a difference in relative position in a pupil division direction) between two image signals obtained by sensing light fluxes passing through different exit pupil areas in an image pickup optical system. Therefore, when a charge accumulation operation is performed once, the amount and the direction of the deviation of focus are obtained at the same time, which allows it to achieve a high-speed focus adjustment operation. In particular, when an image sensing plane phase difference detection method (image sensing plane phase difference AF), in which the phase difference detection is accomplished in an image sensing plane, is employed, it is possible to achieve high-speed and high-quality AF even in a LV (live view) image capture mode or a movie capture mode.
However, in the phase difference AF, various factors may cause a reduction in focus detection accuracy. To handle this situation, it has been proposed to use hybrid AF that allows it to obtain advantages of both contrast AF and phase difference AF. Japanese Patent Laid-Open No. 2013-254166 discloses a focus adjustment technique in which the focus detection method is switched between the phase difference AF and the contrast AF based on object information extracted from a contrast AF signal.
However, in the technique disclosed in Japanese Patent Laid-Open No. 2013-254166, even when the phase difference AF is selected based on the contrast AF signal, a result of the focus detection of the phase difference AF may include an error caused by various factors. Examples of such factors include a change in an image pattern of an object, a change in a range in which an object is captured, which may occur when the object moves, and the like. When the result of the focus detection by the phase difference AF includes an error, if a high-speed focus adjustment operation is performed, there is a possibility that a fluctuation in focus occurs, that is, an in-focus state and a slight deviation from an in-focus point may alternately occur. On the other hand, in a situation in which an object crosses an area in which a main object is captured, if a high-speed focus adjustment operation is performed, there is a possibility that it is tried to achieve an in-focus state for a non-intended object, which may cause a fluctuation in focus.